The Endocrine System

The Endocrine System: An Overview

• A system of ductless glands• Secrete messenger molecules called hormones

• Interacts closely with the nervous system

• Endocrinology – study of hormones and endocrine glands

Endocrine Organs

• Scattered throughout the body

• Pure endocrine organs • Pituitary, pineal, thyroid, parathyroid, and adrenal

glands

• Organs containing endocrine cells• Pancreas, thymus, gonads, and the hypothalamus

• Richly vascularized

Location of the Major Endocrine Glands

Hormones

• Classes of hormones• Amino acid-based hormones

• Steroids

• Basic hormone action• Circulate throughout the body in blood vessels

• Influences only specific tissues – target cells

• A hormone can have different effects on different target cells

Control of Hormones Release: Three Mechanisms

Control of Hormones Secretion

• Always controlled by feedback loops• Blood concentration declines below a minimum

• More hormone is secreted

• Blood concentration exceeds maximum• Hormone production is halted

Structure of the Ovary

The Uterine Wall

hypothalamus

anterior pituitary

FSH LH midcycle peak of LH (triggers ovulation)

Blood levels of FSH (purple) and LH (lavender)

FSH LH LH

estrogens progesterone, estrogen

estrogens progesterone, estrogen

estrogens (light blue) progesterone (dark blue)

growth of follicle

FOLLICULAR PHASE OF MENSTRUAL CYCLE

LUTEAL PHASE OF MENSTRUAL CYCLE

menstruation

endometrium of uterus

ovulationcorpus luteum

GnRH